Generally, high intensity discharge lamps include a pair of spaced electrodes disposed within a sealed envelope and surrounded by a gas which emits light when a discharge takes place between the electrodes. Commonly, the electrodes are embedded in a tube of a material such as quartz and extend through a flattened portion and into an envelope by way of what is usually referred to as a "pinch" seal.
Often such high intensity discharge lamps are of the double-ended construction wherein an electrode is embedded in each one of a pair of tube portions oppositely disposed with respect to a centrally located envelope portion. The envelope portion is formed by a pair of "pinch" seals and each of the electrodes extends through a "pinch" seal into the envelope. However, there are numerous applications where it is much more economical and efficient to provide a single-ended construction wherein a pair of electrical conductors are spaced from one another, embedded in a tube portion and extend through a single "pinch" seal into an envelope having a gas fill therein.
Ordinarily, a discharge between the electrodes is effected by applying a suitable high voltage pulse potential across the electrical conductors which extend into the envelope and serve as electrodes. It has been found that a lamp which has been turned off for a period of time is relatively easy to start since the pressure within the envelope of a relatively cool lamp is usually less than an atmosphere. Thus, a "cold start" voltage of about 2 to 10 KV is not uncommon for a low wattage high intensity discharge lamp. However, it is known that a lamp which has been operational for a period of time has a pressure therein of several atmospheres. Thus, activating a discharge lamp which has recently been turned off requires a relatively high "hot restrike" voltage and voltages in the range of about 35 to 50 KV are not uncommon when energization of a recently operated lamp is desired.
Accordingly, it can readily be understood that problems relating to undesired arcing may be encountered when a single-ended high intensity discharge lamp is subjected to a "hot restrike" voltage after a period of operational use. In other words, a single-ended discharge lamp wherein a pair of electrical conductors are spaced from one another but enter the lamp from the same end are succeptable to undesired arcing, external of the envelope of the lamp, when relatively high "hot restrike" voltages necessary to activate the lamp are employed.
One known attempt to alleviate the above-described undesirable arcing between electrical leads external to the envelope of a high intensity discharge lamp suggests the insertion of an insulator intermediate the electrical conductors. More specifically, the "pinched" end of a glass tube has a pair of spaced electrical conductors embedded therein, a saw cut is made in the tube intermediate the electrical conductors and a piece of mica is inserted into the saw cut of the tube. However, the cutting of a "pinched" tube is not an advisable solution to such a problem because of the tendency for the "sawed" tube to develop undesired and catastrophic crack failures.